Make disconnected overlay sizing styles stronger

[nstrayer]

If you have an app with a rather innocent line of css that restricts sizes of children of the body such as: body > div { max-width: 400px; max-height: 400px; } this will cause the disconnected overlay div to be too small as well.

This would be fine if it still covered the children, but because of the styles of left, right, top, bottom being set explicitly it will always be stuck to the upper-left of the viewport. Here's a minimum app example where the aforementioned styles were used to make the content horizontally centered on the screen and not allowed to get too big.

library(shiny)
shinyApp(
  ui = fluidPage(
    tags$head(
      tags$style(
        HTML("body > div { max-width: 400px; max-height: 400px; }
              #content { margin: 0 auto; background: salmon; }")
      )
    ),
    div(id = "content", "Here's my content")
  ),
  server = function(input, output) {}
)

Current behavior

If we run this app and then the disconnected overlay gets triggered (by simply killing the app process) we get this rather confusing result:

New behavior

And here's the result after the changes in this PR:

[CLAassistant]

All committers have signed the CLA.